The present invention relates to a magnetic disk device for use as a hard disk drive, a removable hard disk drive, or the like, and more particularly to a magnetic disk device capable of generating a common servo clock signal by referring to reproduced signals from a plurality of servo areas thereby to generate a tracking control signal even with short servo areas, so that a number of servo areas can be produced to achieve a sufficient tracking control capability and a sufficient recordable capacity, and the rotational speed of a magnetic disk can be reduced to record moving-image information.
Heretofore, magnetic disk devices exemplified by hard disk drives and removable hard disk drives have been designed for a reduced access time and an increased recording density.
Specifically, the access time is a period of time required for a magnetic head to seek a target track and start recording desired data in or reproducing desired data from the target track. With the magnetic disk devices of the type described above, the access time is expressed as the sum of a seek time for the magnetic head and a rotational delay time (on the average, 1/2 of the time required for the magnetic disk to make one revolution).
In the hard disk drives, the magnetic disk is rotated at a constant angular velocity (CAV) so that the rotational speed of the magnetic disk can be kept constant even when the magnetic head is actuated to seek a target track for thereby effectively avoiding an increase in the access time. In addition to the rotation of the magnetic disk at a constant angular velocity, the hard disk drives rotate the magnetic disk at a high speed for further reducing the access time.
To increase the recording density of magnetic disks, a linear recording density in the longitudinal direction of tracks is increased, and the track density is increased to increase an area recording density. Recent years have seen magnetic disks having an area recording density in excess of 3 Gbit/inch.sup.2.
If the rotational speed of a magnetic disk is increased and the recording density thereof is increased, then the data transfer rate of the hard disk drive is increased. Specifically, in a 3.5-inch hard disk drive having a linear recording density in excess of 200 kbit/inch, the rotational speed of the disk is about 5400 rpm, and the maximum data transfer rate is higher than 170 Mbit/sec.
In recent years, some hard disk drives are designed to record image data. According to the MPEG (Moving Picture Experts Group) 2 standards, moving images of sufficient quality can be displayed if the image data recorded by hard disk drives are transferred at an average data transfer rate ranging from 4 to 8 Mbit/sec.
Consequently, the conventional hard disk drives have an unnecessary high data transfer rate for recording and reproducing moving-image data.
As described above, the data transfer rate depends on the rotational speed of the magnetic disk. If the unnecessary high data transfer rate is reduced to lower the rotational speed of the magnetic disk, then the power consumption and noise of the hard disk drives can advantageously be reduced.
If the rotational speed of the magnetic disk is lowered, however, the accuracy with which to position the magnetic head is reduced, and the track density is lowered.
Specifically, conventional hard disk drives have servo areas positioned at certain angular intervals on the information recording surface thereof and data areas for recording data which are positioned between the servo areas. There are several tens of servo areas per track, which record positional information required to position the magnetic head, such as head positions, track numbers, etc., and synchronizing patterns necessary for acquiring the positional information.
In each of the servo areas, after a clock signal is synchronized, positional information of the magnetic head is acquired, and tracking control is performed on the magnetic head according to the acquired positional information. The magnetic head is positioned by a servo loop generated in a magnetic head positioning system. The magnetic head positioning servo system determines a closed-loop control frequency band Bsv under various conditions.
The hard disk drives are required to position the magnetic head centrally on tracks irrespective of various disturbances. The disturbances caused within the hard disk drives include periodic and aperiodic components of radial displacements of the shaft of a spindle motor, and positional deviations of the magnetic head due to vibrations of the magnetic disk and vibrations of a head support arm. The disturbances from outside of the hard disk drives include various vibrations and shocks. These disturbances have a frequency spectrum in a low frequency range below 1 [kHz] In order to sufficiently suppress the effect of these disturbances, the closed-loop control frequency band Bsv of the magnetic head positioning servo system needs at least several hundreds [Hz].
Generally, the control frequency band Bsv of a positioning system for a magnetic disk or the like is required to be wider in proportion to the one-half power of the track density TPI according to the following equation (K. K. Chew "Control system challenges to high track density magnetic storage" IEEE Trans. Magn., Vol. 32, No. 3, pp. 1799-1804, May 1996): EQU Bsv.varies.(TPI).sup.1/2 (1)
Therefore, the control frequency band Bsv of the positioning system for a magnetic disk or the like needs to be as high as the present hard disk drives which have the unnecessary high data transfer rate.
On the other hand, since the servo system of the type described above is a closed-loop sampling control system, its servo sampling frequency fsv needs to be at least 10 times higher than the control frequency band Bsv in order to achieve control loop stability. Stated otherwise, the servo sampling frequency fsv needs to be related to the control frequency band Bsv according to the following equation: EQU fsv&gt;10Bsv (2)
At present, the servo sampling frequency fsv is set to at least several kHz.
The servo sampling frequency fsv represents the number of servo areas scanned by the magnetic head per unit time, and is expressed by the product of the number Nsv of servo areas along a track on the magnetic disk and the rotational speed R of the magnetic disk, according to the following equation: EQU fsv=Nsv.multidot.R (3)
From the equations (1) through (3), the following equation (4) is derived. It can be seen from the equation (4) that when the rotational speed R of the magnetic disk is lowered to a level necessary and sufficient to record and reproduce moving-image information, the control frequency band Bsv is also lowered. EQU Nsv.multidot.R=10Bsv.varies.(TPI).sup.1/2 (4)
If the control frequency band Bsv is lowered, the ability of the magnetic disk device to suppress the effect of various disturbances is lowered, reducing the accuracy with which to position the magnetic head, so that tracks cannot be formed at a high density.
One solution to the above problem would be to increase the number Nsv of servo areas per track on a magnetic disk. On conventional magnetic disks, however, servo areas are relatively long because of the need for resynchronizing the clock signal in each of the servo areas. Therefore, if the number Nsv of servo areas were increased, then the recordable capacity of the magnetic disk would greatly be reduced.